The aerospace industry has consistently driven the leading edge of technology with its requirement for lightweight, high efficiency, high reliability equipment: lightweight because each additional pound of weight translates directly into increased fuel burn, and therefore, a higher cost of ownership and shorter range; high efficiency because each additional cubic inch required for equipment displaces the amount of revenue generating cargo and passengers that can be carded on an aircraft; high reliability because every minute of delay at the gate increases the cost of ownership, and likewise, increases passenger frustration. As equipment efficiency is driven to increase, and component size is driven to decrease, problems arise which must be overcome.
For the electric power generation systems for use on aircraft, these pressures have precipitated great advancements in technology, and have also precipitated problems which must be overcome before this new technology can be applied. Aircraft have typically used synchronous brushless AC generators or permanent magnet generators for their electric power generation needs. Unfortunately, both of these types of generators, which have been the workhorse of the aerospace electric power system, require components which often fail due to the harsh environment in which they are required to operate (usually mounted directly on the aircraft jet engine). The synchronous AC generators, for example, have rotating rectifiers which often fail due to the harsh environment; this decreases reliability. Both types of generators require some form of rotor containment due to the speed at which they operate; this increases weight. Additionally, both the brushless AC generators and the permanent magnet generators have stator coil windings which physically overlap, resulting in the potential damage of the wire insulation during the assembly process; this also decreases reliability.
In addition to an electrical generator, an engine starter is also typically installed on the aircraft engine. While this starter is important to start the engine, it becomes excess baggage during the remainder of the flight, increasing overall weight, increasing required fuel burn, increasing cost of ownership, and decreasing overall range. This problem has been recognized and efforts to combine the starter and generator into a single package, thus eliminating the need for an additional piece of equipment used only a fraction of a percent of the time, have proceeded. Unfortunately, using synchronous AC or permanent magnet generators for this purpose, in addition to creating new problems associated with the start function, does not eliminate the inherent problems with these machines as described above.
As a alternative to the use of the synchronous AC or the permanent magnet generator for this combined starter/generator function, a switched reluctance machine can be used. A switched reluctance machine is an inherently low cost machine, having a simple construction which is capable of very high speed operation, thus yielding a more lightweight design. The rotor of the switched reluctance machine is constructed from a simple stack of laminations making it very rugged and low cost. The rotor does not require rotating rectifiers, which are a large source of failures, as does the AC synchronous machine. As a further consequence of this simple rotor construction, the machine is capable of very high speed operation without the containment problems associated with rotor windings or permanent magnets.
Switched reluctance machines, however, require that careful design consideration be given to the cooling of the stator windings and the rotor structures themselves. The stator coil winding electrical losses consist of the DC I.sup.2 R component, plus the eddy current components that are proportional to the electrical operating frequency. Because the switched reluctance machine is capable of operating at a very high speed, the frequency is thus very high, and therefore, the eddy current component is high. Furthermore, the magnetic field distribution of the stator coils is not uniform when the rotor pole is not aligned with the stator pole. The conductors near the air gap are exposed to higher magnetic flux density. The consequence of this is that the conductor adjacent to the stator pole has the highest power loss. Compounding the physical location of the coil windings, the winding configuration of the stator coils also results in less cross-sectional area for heat transfer through the stator laminations. Cooling of the stator windings, therefore, becomes a major problem with switched reluctance machines.
The rotor of a switched reluctance machine, as indicated above, is made simply from iron laminations which yield a robust configuration ideally suited for high speed applications. To ensure a robust mechanical design of the rotor, one must properly account for its electromagnetic losses. These rotor losses are a result of the non-sinusoidal flux distribution in the laminations. Eddy current and hysteresis losses amount to a substantial portion of those associated with the rotor. Combined, these losses result in a temperature rise in the rotor laminations which tends to decrease the mechanical yield strength of the lamination material. This presents a substantial problem if not properly dealt with.
Others have attempted to solve these problems to allow efficient use of a switched reluctance machine as a combined starter/generator for an aircraft. One such system is disclosed by Stephen MacMinn and William Jones in a paper entitled "A Very High Speed Switched-Reluctance Starter-Generator For Aircraft Engine Applications," published as part of the proceedings of NAECON '89, National Aerospace Electronics Conference, May 22-26, 1989, Dayton, Ohio. In the system presented therein, MacMinn attempts to address the cooling problem of the stator windings by flowing oil in non-magnetic cooling tubes placed at the top of the stator slots. Although this increases the cooling efficiency over strictly relying on heat transfer through the cross sectional area of contact with the stator poles, it does not efficiently remove heat from all of the windings. Those windings which do not touch the cooling tubes still must rely on heat transfer through the cross sectional area of contact with the stator poles or other windings. The rotor heat is removed by utilizing a hollow shaft through which oil is flowed. Although this effectively removes the heat from the rotor, it does so in a very inefficient manner. Heat is only removed by the oil which contacts the inside surface of the shaft. By flowing oil through the entire void of the shaft, a majority of the oil serves no useful purpose, and in fact increases the system weight by requiting a higher volume of oil to be carried in the system.